The present invention relates to a method and an apparatus for detecting a wave front caused by the onset of leaks or other transient events in a pipeline and, more particularly, to a method and an apparatus for making use of measurements of the pressure of a fluid in the pipeline to determine the arrival time and/or amplitude of the wave front.
It has been long known that most pipeline operational events such as operation of valves, start-up or shut-down of pumps, or a break in the pipeline can result in a sudden pressure change at the location of the event. This pressure transient immediately forms a pressure wave which travels both up-stream and down-stream at the speed of sound in the fluid flowing in the pipeline. Were it not for the fact that pipelines typically are highly turbulent, energetic, dynamic systems with continuously changing pressure conditions, these pressure events could be easily detected and used by pipeline operators to confirm commanded operations of valves or pumps, or to detect undesired leaks or surges.
Because of the significant amount of randomly generated pressure noise found in most pipelines, the use of the leading edge of a pressure wave (i.e. a wave front) has typically been unsuccessful or impractical. It is highly useful to detect the wave front. For example, the time of its arrival at a detector can be used to determine the location of an event along the pipeline which caused the pressure wave. The derived location can be correlated against apparatus on the pipeline to confirm occurrence of desirable operations, such as that of a valve, or to locate a leak so that appropriate remedial action can be taken. One way of computing the location from such time of arrival is disclosed in U.S. Pat. No. 3,851,521 issued to Ottenstein. Amplitude data on the wave front can also be derived. This is valuable for providing an indication as to the size of a leak caused by a break in the pipeline so that a decision can be made on the nature and urgency of the required remedial action. Furthermore, the location of the event can also be determined from the amplitude. One way of computing the location from such amplitude is disclosed in EPO published application no. 0 042 212.
Examples of prior art methods intended to detect arrival of the wave front are disclosed in U.S. Pat. Nos. 4,144,748, 4,091,658 and 4,012,944 issued to Covington and in U.S. Pat. No. 4,796,466 issued to Farmer. The inventors of these prior art approaches found that the leading edge of the pressure drop signals was too difficult to reliably detect when obscured by the turbulent background pressure noise and, therefore, they chose to heavily filter the pressure signals until a significant trend in the pressure signal profile which follows the leading edge could be established.
The application of a single filter, such as the electrical integrator of Covington or the statistical filter of Farmer, eliminates some noise and can help identify a trend. However, when these filters are set to time constants long enough to be useful (e.g. an upper cutoff frequency of 5 Hz), they also destroy all evidence of the leading edge of the wave front by averaging these rapidly changing pressures into slowly changing values. This makes it virtually impossible to accurately determine the arrival time and amplitude of the passing wave front.